Pepsin, was, in the 18th century, the first enzyme to be discovered and the second enzyme to be crystalized. It is an enzyme created by the stomach expressely for breaking down proteins. Now, the problem arises as to how to create an enzyme for breaking down proteins without destroying the cell that creates it, which is largely protein based. Pepsin is created with an extra 44 amino-acidsbonded to its active site, in a form called pepsinogen. Once pepsinogen is escorted out of the cell and into the highly acidic digestive fluids, HCL acid cuts off the extra 44 amino-acids, thereby activating pepsin to do its duty of breaking apart proteins.

Stats:

Molecular Weight: 35,000+ AMUpH optimum: 1 - 4
Stability: Pepsin is stable up to a temperature of 60 degrees Celsius, and is irretrivably deactivated at a pH of 6
Pepsin types: A,B,C,D, With A being a supermajority.
Pepsin will not cleave at places bonded by valine, alanine or glycine.

An unorganized proteolytic ferment or enzyme contained in the secretory glands of the stomach. In the gastric juice it is united with dilute hydrochloric acid (0.2 per cent, approximately) and the two together constitute the active portion of the digestive fluid. It is the active agent in the gastric juice of all animals.

⇒ As prepared from the glandular layer of pigs' or calves' stomachs it constitutes an important article of pharmacy.